Modification of the growth and the competitiveness of a Bradyrhizobium strain obtained through affecting its siderophore-producing ability

Abstract

Under iron-deficient conditions, strains of Rhizobium and Bradyrhizobium secrete iron-binding compounds (siderophore) to acquire iron. The objectives of the work were to evaluate the competitiveness for nodulation and the growth of a siderophore-producing Bradyrhizobium wild-type strain (sid+ parental strain) and those of its siderophore non-producing mutant (sid- mutant) in the rhizosphere of two provenances of Acacia mangium cultivated under iron-limited condition and also in the nonrhizosphere zone without any iron supplementation. In the nonrhizosphere zone, population of the sid- mutant decreased markedly compared to that of the sid+ parental strain when the bacteria were alone. This suggests that the lack of iron in the milieu was more harmful to the sid- mutant than to the sid+ parental strain. However, in mixed inoculation experiments, the decrease of the sid- mutant’s population indicated above was significantly reduced suggesting that probably this strain might take benefits from the iron-binding siderophore presumably secreted by the parental strain. In the rhizosphere, growth of both strains was apparently stimulated similarly by the root exudates from the two provenances of Acacia mangium studied here. Scrutiny of the R/S ratio however showed that at the early growth of the plants, provenances Iron Range and Dimisisi preferentially favoured growth of the parental strain and the mutant respectively. Consequently, it was found that when inoculating plants of the Iron Range provenance, the sid+ parental strain was more competitive than the sid- mutant as expressed by nodule occupancy. Inversely, the sid- mutant seemed to be more competitive than the sid+ parental strain when the provenance Dimisisi was used. Our investigation therefore suggests that: 1) siderophore-producing ability may favour the persistence of Bradyrhizobium in iron-deficient soil; 2) the sid+ parental strain and its sid- mutant exhibited different growth responses and competitiveness according to the host-plant provenances whose role should not be neglected.